This invention relates to heat transfer sheets and more particularly is intended to provide a heat transfer sheet capable of easily producing a recorded image of excellent fastnesses onto a heat transferable material.
Various heat transfer methods have been known in the art, among which the sublimating transfer method has been and is being practiced. In this method a sublimating dye is used as the recording agent, which is carried on a substrate sheet such as paper, etc. to provide a heat transfer sheet, which is then superposed on a heat transferable material dyeable with a sublimating dye such as a fabric made of polyester, and heat energy is imparted in a pattern from the back surface of the heat transfer sheet to cause the sublimating dye to migrate to the heat transferable material.
In the case of this sublimating transfer method, in the sublimating printing method in which the heat transfer material is, for example, a fabric made of polyester, the heat transferable material itself is also heated by the heat energy imparted since heat energy is imparted for a relatively longer time, whereby relatively good migration of the dye can be accomplished.
However, with the progress of the recording method, by the use of, for example, a transferable material having a dye receiving layer provided on a polyester sheet or paper and by the use of a thermal head at high speed when fine letters or figures or photographic images are to be formed on these transferable materials, heat energy must be imparted within an extremely short time of the order of seconds or less, and the sublimatable dye and the transferable material cannot be heated sufficiently within such a short time, whereby an image with sufficient density cannot be formed.
Accordingly, in order to correspond to such high speed recording, sublimating dyes of excellent sublimatability have been developed. However, dyes of excellent sublimatability generally have smaller molecular weights, and therefore there arise problems such as migration of the dyes in the heat transferable material after transfer and bleeding of the dyes onto the surface, whereby the images formed with much effort may be distorted or become unclear or may contaminate surrounding articles.
When a sublimating dye with relatively greater molecular weight is used for avoiding such problems, an image with satisfactory density could not be formed because of inferior sublimation speed according to the high speed recording method as described above.
Accordingly, in the method of heat transfer by the use of the sublimating dye, there has been a great demand under the present situation for development of a heat transfer sheet which can produce a clear image with sufficient density and an image formed exhibiting various fastnesses.
We have carried out intensive studies in order to respond to the strong demand in the field of the art as described above. As a result, in the light of the art of the sublimating printing method of fabrics made of polyester, etc., in which due to non-smoothness of the surface of the fabric, the heat transfer sheet and the fabric which is the heat transferable sheet are not sufficiently contacted, and therefore the dye to be used is essentially required to be sublimatable or gasifiable (namely migratable through the space existing between the heat transfer sheet and the fabric), it has been found that in the case of using a polyester sheet or surface processed paper, etc., with smooth surface as the heat transferable material, the heat transfer sheet and the heat transferable material can sufficiently contact each other, whereby only the sublimatability or gasifiability of the dye is not an absolutely necessary condition, but the property of the dye migratable through heat between the closely contacted interface of both is also extremely important, and such heat migratability at the interface is greatly influenced by the chemical structure of the dye used, the substituent or its position. Thus, it has been found that even a dye with a high molecular weight as generally accepted in the prior art as unuseable has good heat migratability by selecting a dye having an appropriate molecular structure. And by the use of a heat transfer sheet having such a dye carried thereon, it has been found that the dye used can be caused to migrate easily to the heat transferable material to form a recorded image with high density and various excellent fastnesses.